Liquid crystal display devices have been widely used in various devices, such as liquid crystal television displays, monitors, mobile phones, etc., as flat panel displays which have various advantages, such as slim body, light weight, etc. However, one type of the liquid crystal display devices which presently has the widest applications employs two or one polarizing plate so that, disadvantageously, the light utilization efficiency is low.
Conventionally, as the display type which does not employ a polarizing plate, the guest-host type and the Polymer Dispersed Liquid Crystal (PDLC) type have been proposed (for example, Patent Documents 1 and 2).
Patent Document 2 discloses a PDLC-type liquid crystal display device which has improved scattering efficiency. A liquid crystal layer of the PDLC-type liquid crystal display device (PDLC layer) includes a plurality of liquid crystal regions (or “liquid crystal droplets”) dispersed in a polymer material. The liquid crystal regions are formed in spaces defined by polymer walls (hereinafter, referred to as “small sections”). In the PDLC having such a structure, when no voltage is applied (i.e., in the absence of an applied voltage), there is a difference in refractive index between the liquid crystal in the liquid crystal regions and the polymer, so that light is scattered by the interfaces between the liquid crystal regions and the polymer, resulting in a white display state. When a voltage is applied across the PDLC layer (i.e., in the presence of an applied voltage), the alignment of the liquid crystal changes so that the liquid crystal and the polymer have generally equal refractive indexes, allowing light to be transmitted through the PDLC layer without being scattered. If, in this situation, there is a light-absorbing plate on the rear side of the liquid crystal layer, the transmitted light is absorbed by the light-absorbing plate, resulting in a black display state. The entire disclosures of Patent Document 2 are incorporated by reference in this specification.
Patent Document 1 discloses a PDLC-type liquid crystal display device which includes a dichromatic dye. Specifically, this liquid crystal display device includes an upper polymer-LC composite film and a lower polymer-LC composite film in which a nematic liquid crystal material and a dichromatic dye are dispersed in gaps of the polymer. In each of the polymer-LC composite films, the nematic liquid crystal material is aligned in a predetermined direction in a plane which is generally parallel to the electrode plane. The alignment azimuth of the nematic liquid crystal material in the upper polymer-LC composite film and the alignment azimuth of the nematic liquid crystal in the upper polymer-LC composite film are set so as to form an angle of 90° with respect to each other. It is disclosed that the liquid crystal display device having such a configuration is capable of achieving a required contrast ratio and a reduced driving voltage even when the cell gap of the liquid crystal display device is narrowed.